J. Phys. Chem. 1980, 84, 817-819 Scientific Research Laboratory, Ford Motor Company, Dearborn, MI 48121. E. J. Hamilton, Jr., J . Chem. Phys., 63, 3682 (1975). E. J. Hamilton, Jr. and R. R. Lii, Inf. J . Chem. Kinet., 9, 875 (1977). W. B. DeMore, J . Phys. Chem., 83, 1113 (1979). R. A. Cox and J. P. Burrows, J. Phys. Chem., 83, 2560 (1979). P. 8. Pagsberg, J. Eriksen, and H. C. Christensen, J . Phys. Chem., 83, 582 (1979). R. R. Lii, R. A. Gorse, Jr., M. C. Sauer, Jr., and S. Gordon, J . Phys. Chem., 83, 1803 (1979). S. Gordon, W. Muhc, and P. Nangia, J. phys. Chem., 75, 2087 (1971).
Rate Conistant of the Reaction of O(3P) with
817
(11) J. White, J . Opt. Soc. Am., 32, 285 (1942). (12) The back-off circuit was designed by C. Jonah, and details can be obtained from the authors. (13) T. T. Paukert and H. S. Johnston, J. Chem. Phys., 56, 2824 (1972). (14) C. J. Hochanadel, J. A. Ghormley, and P. J. Ogren, J. Chem. Phys., 56, 4426 (1972). (15) P. S.Nangia alnd S. W. Benson, J . Phys. Chem., 83, 1138 (1979). (16) S. W. Benson, “Thermochemical Kinetics”, Wiley, New York, 1968, Appendix. (17) E. J. Hamilton, Jr. and C. A. Naleway, J. Phys. Chem., 80, 2037 (1976).
Hop’
Ruey-Rong Lii,‘ Myran C. Sauer, Jr.,* and Sheffleld Gordon Chemistry Division, Argonne National Laboratory, Argonne, Illinois 80439 (Received October 4, 1979) Publication costs assisted by Argonne National L.aboratofy
The rate constant for the reaction of O(3P)with H02 is measured at 298 K by spectrophotometricobservation of the H02 decay (230 nm) and the O3 formation (265 nm) following irradiation of systems of ca. 3.6 >< 10’’ molecules of Ar ~ m -6.5 ~ ,X lOls molecules of H2cm-3, and 1.6 X 101a-6.5X 1Ol8 molecules of 0 2 cm-3with electron pulses. The rate constant obtained is (7 f 2) X lo-” cm3 molecule-l s-l.
Introduction One of the reactions of importance in the interconversions among hydrogen-containing species in the stratosphere is 0 HOz OH .t 0 2 (1)
+
-+
We have been using the pulse radiolysis technique, which involves kinetic spectrophotometry, to determine rate constants for reactions of H02.3-6 Application of these techniques to a chemical system designed to maximize the importance of reaction 1has enabled us to measure its rate constant, thereby adding to the limited amount of experimental information on this r e a ~ t i o n . ~
Experimental Section The general characteristics of the apparatus and techniques employed have been described ear lie^-.^,^ Improvements in the method have been detailed in a more recent publication.5 Hydrogen (99.999%), oxygen (99.99%), and argon (99.999%)from Mathesson were used without purification. All experiments were carried out at 25 f 0.5 “C. Oxygen (50,100, or 200 torr) was introduced into the cell, followed by 200 torr of H2. Ar was then added to bring the total pressure up io 1200 torr (3.9 X 1019molecules ~ m - ~The ). gases were allowed to mix in the cell for at least 10 min before the electron pulse. Two sets of experiments were carried out. In one, H02 decay was monitored spectrophotometrically at 230 nm; in the other, O3 formation was monitored at 265 nm. For obvious reasons, it is important to note here that an explosion occurred upon irradiation of 200 torr of 02, 200 torr of Hz, and 800 torr of Ar. The explosion occurred simultaneously with the first pulse given the sample. An identical sample had been irradiated with two pulses with no explosion, Results and Discussion The absorbance was measured as a function of time after the pulse at 230 and 265 nm. At the former wavelength
HOz and O3 are the main absorbing species, with HzOz contributing slightly to the absorbance. At 265 nm O3 dominates the absorbance, with small contributions from H 0 2 and H202. The results are shown in Figures 1 and 2 for the systems described in the legends. The sequence of events following the pulse is as follows: (1)the recombination of ions and disappearance of excited species is very rapid; hence, only neutral atom and radical species need be considered after about 1 ps; (2) the high concentration oE oxygen and the “third body” causes all hydrogen atoms to react with O2 within about 2 ps, Le., before the hydrogen atoms have a chance to react with other species and also before HOz undergoes appreciable reaction (using known values for the three-body H + O2 reactiong). Therefore, on the time scale of these experiments, the simplifying assumption can be made that we have only H02 and O(3P)as “initial” reactants. The O(3P) is expected, on the basis of known rate constants? to react with O2 to form ozone with a tl of about 20 ps at 1.6 >( 1Ol8 molecules of O2 ~ m - ~ Under . the conditions of the,,?e experiments, the initial concentrations of H02and O(3P)are approximately2 X 1015molecules cm-3 (aswill be seen from the analysis of the observed H02and O3 absorbances), and reaction 1 will therefore proceed at a rate comparable to that of the ozone-formation reaction if hl is approximately 5X cm3 molecule-I s-l. In other words, if the latter value of lzl is approximately correct, the experimental conditions should be such that its value can be determined with reasonable precision. The experimental results were fitted in terms of the mechanism and rate constants given in Table I, with kl as a variable (fit) parameter. For each concentration of oxygen, the initial concentrations of H02and O(3P)were also treated as variable parameters. Known values of the absorption cross sections (in cm2molecule-l, base e ) used are the following: H02, ~ ~ 2 3=0 2.17 X 10-18,10g 2 6 5 = 1.1X 10-19;1103,~ 2 3 0=: 4.49 x u 2 6 5 = 9.47 x 10-1s;12 and HzO2, U p = 2.21 X io-’’, 0 2 6 5 = 5.9 X 10-20.’3 The fitting procedure involved an iterative, nonlinear least-squares computer method in which the differential
0022-3654/80/2084-0817$01.00/00 1980 American Chemical Society
818
The Journal of Physical Chemistry, Vol. 84, No. 8, 1980
Lii et al.
TABLE I: Kinetic Model reaction no.
OtHO,-OHtO, HO, t HO, -+ H,O, t 0, OH t HO, H,O t 0,
4 5 6
oto , ~ o ,
7 8 9 10
......
0
I
I
L
05
0
L
1J
-
15
IO TIME (ins)
Flgure 1. Absorbance (log [ I o / I v ] )at 230 nm vs. time. Experimental: (0)1.6 X IO’’ molecules of O2 ~ m - ~(X); 3.2 X molecules of O2cm-? (0) 6.5 X 10” molecules of 0,~ m - ~ The . concentration of H, in all cases was 6.5 X 10“ molecules ~ m -and ~ , the sum of the Ar and 0,concentrations was 3.9 X IO1’ molecules ~ m - The ~ . curves are the result of least-squares fitting (see text): (-) best fit (k,= 4.8 X lo-‘’ cm3 molecule-l~-~, see text); (--) fixed kl at 3.2 X IO-’’ cm3 molecule-is-l; (---)fixed kl at 7.3 X IO-” om3 molecule-’ Si.
E LD
N W
4 c W 0
z
7+2 0.31 10.0
b 5 6
O t OH-+HOZd OH t 0, HO, t 0,
3.8 0.0089
9 9 14
OH+ O H ~ H , O , OHtOH-+H,O+O OH t H,O, HO, t H,O OH t H, H,O t 0
0.83 0.22 0.083 0.0006
e 9 9 9
C
-+
-+
-+
a Temperature 298 K. Determined by the fitting procedure; see text. k , = (6.2X 1 0 - 3 4 ) [ H 2 ]t (6.2X 1 0 - 3 4 ) [ 0 2t] (3.65X 10-34)[Ar], in cm3 molecule-’s-’. The H, third-body efficiency is assumed to be equal to that of 0,. Reaction 5 actually produces H t 0,, but H can be assumed to react instantly with 0, (see text). e We have justified the use of this value, which is somewhat higher than expected from the literature, in ref 6; however, a value of 0.5 X lo-” can be used without significantly changing the results (see text).
TABLE 11: Results of Least-Squares Fittinga POL8[ O21, 101lkl, cm3 loL5[ HO, I,, lo”[ q 3 P ) l 0 , molecule molecule-’ molecule molecule ~rn‘~ scm-3 cm-3 4.8 4.8 4.8 7.3b 7.3b 7.3b 3.2b 3.2b 3.P
1.76 2.00 2.57 1.87 2.13 2.64 1.68 1.91 2.52
0.70 0.91 1.31 0.88 1.14 1.59 0.55 0.75 1.12
a The standard deviations in the values of [ HO, ] and The value of k , [O(,P)] were in the range of 5-10%. was fixed.
3 005
4
C
-+
1.6 3.2 6.5 1.6 3.2 6.5 1.6 3.2 6.5
0 10
5m
reactiona
1 2 3 .....................
m
10” x rate constant, cm3 molecule-’ s-’ ref
I
L
3
1
I
GI
02
03
04
TIME ( m s )
Figure 2. Absorbance (log [ I o / I k ] )at 265 nm vs. time. Experimental: (0)1.6 X IO’* molecules of 0,~ m - ~ ( X;) 3.2 X 10” molecules of O p~ m - ~(0) ; 6.5 X lo1’ molecules of 0,~ m - ~ The . concentration of H, in all cases was 6.5 X IOi8 molecules ~ m - and ~ , the sum of Ar and O2concentrations was 3.9X IO’’ molecules ~ m - The ~ , curves are the result of the least-squares fitting (k, = 4.8 X IO-” cm3 molecule-‘ s-I, see text).
equations corresponding to the kinetic model were numerically integrated. The best fit is shown by the solid curves in Figures 1and 2 and corresponds to hl = (4.8 f 1.1)X cm3 molecule-I s-l, the error limits being two standard deviations. As a check on the sensitivity on the fit to the value of hl, the value of hl was fixed at 7.3 X and 3.2 X and the “best values” of the concentration parameters were determined for each of these rate constants. The short-dash and long-dash curves in Figure 1 correspond, respectively, to these values of hl. (The calculated Os-formation curves were not markedly affected by variation of hl over this range; therefore, only the curves for the “best-fit’’ value of kl are shown in Figure 2.) The results for the initial concentrations of HOz and O(3P)for
the three values of hl are given in Table 11. The “best-fit’’ value of hl results in a calculated curve, for the case of [O,]= 6.5 X l0ls molecules ~ m -which ~ , is in qualitative disagreement with experiment because it shows the absorbance at 230 nm increasing to a maximum a t early times (-70 ps). This was not observed expericm3molecule-’ mentally; hence, a value of hl N 7.3 X s-l (dotted curves) is favored by this aspect of the data. The overall fit is not significantly worse when this value of hl is used, and no initial increase in the absorbance a t 230 nm is obtained. The effects of the values of h2, h,, and k7 on the value obtained for hl were investigated. Increasing k2 by a factor of 1.3 caused a 21% decrease in kl,and decreasing k2 by a factor of ‘/1,3 caused a 21% increase in kl. Increasing k 3 by a factor of 1.3 caused only a 3% increase in kl,and decreasing k3 by a factor of 1/1,3 caused a 3% decrease in to 0.5 X hl. Changing the value of k7 from 0.83 X had no effect on the value of kl. In view of the foregoing, a value of kl of about 7 X 10-l’ molecule-I is indicated, with approximate error limits This value is higher than the value of 3.5 of f 2 X x 10-l’ cm3 molecule-I s-l obtained in a previous study7 of reaction 1. A significant difference between experiment and calculation in Figures 1 and 2 concerns the level of absorbance at times greater than about 0.5 ms, where most of the absorbance at both 230 and 265 nm should be due to ozone.
J. Phys. Chem. 1980, 84, 819-821
The nature of the discrepancy is such that ascribing it to an error in the ratio of aO3a t the two wavelengths would not be unreasonable if the spectrum of ozone were not so well established. Since this explanation is not reasonable, we can only attribute the discrepancy t o some unknown source of error. One further aspect of the results which warrants discussion is ths variation of [HO,], and [O(3P)]owith [O,], shown in Table 11. The increase in [O(3P)]owith increasing [02] is expected, but we are unable to present any quantitative explanation as to why [HO,], should increase with increasing [OZ]. There does not seem to be any appreciable “sink” for H atoms other than reaction with Oz under the conditions of‘these experiments. We can only suggest the possibility that a sequence of ion-molecule reactions results in a greater efficiency of utilization of energy for H (or HOJ formation as [O,] is increased.
Acknowledgment. We thank Dr. C. Jonah for the least-squares fitting program.
819
References and Notes ( I ) Work performed under the auspices of the Office of Basic Energy Sciences of thle U.S. Department of Energy.
(2) Department of Chemlstry, Malcolm X College, Chicago, IL 60612. (3) E. J. Hamilton, Jr. and R. R. Lii, Int. J. Chem. Kinet., 9, 875 (1977). (4) R. R. LII, R. A. Gorse, Jr., M. C. Sauer, Jr., and S. Gordon, J . Phys. Chem., 83, 1803 (1979). (5) R. R. Lii, R. A. Gorse, Jr., M. C. Sauer, Jr., and S. Gordon, J . Phys. Chem., preceding paper In this issue. (6) R. R. Lii, R. A. Gorse, Jr., M. C. Sauer, Jr., and S. Gordon, J. Phys. Chem., following paper in this issue. (7) J. P. Burrows, G.W. Harris, and 8. A. Thrush, Nature (London),267,
233 (1977). (8) S.Gordon, W. Mubc, arid P. Nangia, J. phys. Chem., 75, 2087 11971). (9) R. F. Hampsori, Jr. and D. Garvin, Eds., Nat. Bur. Stand. (U.S.), Spec. Pub/., No. 513 (1978). (IO) T. T. Paukert and H. S. Johnston, J. Chem. Phys., 56, 2824 (1972). (1 1) Based on the value at 230 nm from ref 10 and relative values from C. J. Hochanaclel, J. A. Ghormley, and P. J. Ogren, J. Chem. Phys., 56, 4426 (1972). (12) M. Griggs, J. IChem. Phys., 49,857 (1968). (13)J. G. Calvert and J. N. Pitts, Jr., “Photochemistry”, Wiley, New York, 1967,p 201. (14) A. R. Ravishankara, P. H. Wine, and A. 0.Langford, J. Chem. Phys., 70, 984 (1979).
Rate Constant for the Reaction of QH with H02’ Ruey-Rong Lii,’ Robert A. Gorse, Jr,,3 Myran C. Sauer, Jr.,” and !Sheffield Gordon Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received October 4, 1979) Publication costs assisted by Argonne National Laboratory
-
Spectrophotometric observation of both HOz and OH in pulse-irradiated Ar-Hz0-02 systems leads to a value cm3 molecule-I s-l at 308 K for the reaction OH + HOz HzO + 0 2 . of hl = (0.99 f 0.12) X
Introduction There is particular interest in the reaction between OH and HOz (eq 1)from the standpoint of models of reactions OH + HOz HzO + 02 (1) occurring in the stratosphere. Recent investigations4p5of this reaction under the low-pressure conditions typical of discharge flow techniques indicate a value of kl N 3 X cm3 molecule-l s-l, but at higher pressure and in the cm3 presence of water vapor, a value of kl in excess of molecule-l s-l has been determined,6 and the suggestion was put forward that the difference could be related to a pressure effect or water vapor effect (complexing of the HO,). Earlier work on the determination of kl, which is cited in ref 4 -6, shows a similarly wide range of values for
-
kl. Experimental Section The general characteristics of the apparatus and techniques used in this investigation have been described in detail e a r l i e ~ r .Improvements ~~~ in the method have been detailed in a more recent publi~ation.~ Oxygen (99.99%) and argon (99.999%) from Matheson were used without purification. The water was triply distilled and thoroughly degassed. The decays of H 0 2and OH were monitored by kinetic spectrophotometry at 230 nm (spectral resolution 1.7 nm) and 308.7 nm (spectral resolution 0.08 nm), respectively. The light source for 230 nm was a 1-kW Hg-Xe high-pressure lamp. The lamp used a t 308.7 nm was a 450-W xenon lamp, intensified by applying an approxi-
TABLE I: Kinetic Modela reaction no.
reaction
OH t HO,
H,O + 0, 0 + H,O
10” x rate constant, cm3 molecule” s’-’ ref
1 2
OH + OH
3
OH
4
0t 0 : , 3 0 ,
0.00156
110
5 6 8 9 10 11 12
Ht O:,~HO, HO, + HO, 0,t H,O, OH + H,O, HO, t H,O OH + 10, HO, + 0, O t H O , + O H + 0, H + H10, H, + 0, H + H10, -+ OH + OH H + HID, 0 + H,O
0.095 0.45d 0.088 0.0089 7 1.34 1.93 1.64
110 8 10 11 12 10 110 10
13 14
H t O.KsH,O O+OH-+H+O,
1.00 4.0
10 10
7
-+
-+
+ OHM’H,O,
-+ -+
-+
-+
+
b
9.9 0.22
10
0.83
c
a For 3.9 X 10’’molecules of Ar ~ m - 3.9 ~ , x 10’’ molecules of H,O cm-3,and about 1.6 X 10’’molecules of 0, ~ r n at - ~308 K. Determined by the fitting procedure. See text. Thlis value takes into account the known effect of water vapor on reaction 6.8
mately square wave voltage pulse lasting for up to 2 ms. All experiments were carried out at 35 f 0.5 “C. Oxygen gas (2,4,6, or 8 torr) was introduced into the cell, followed by 12 torr of water vapor. Ar was then added to bring the total pressure up to 1200 torr (3.9 x iO19 molecules ~ m - ~ ) .
0022-3654/80/2084-0819$01.00/00 1980 American Chemical Society
